Compensated frequency biasing system for ring laser

ABSTRACT

A compensated Faraday bias cell system is adapted to shift differentially the respective phases of the contradirectionally flowing light waves propagating in a ring laser cavity. The desired phase shift is produced by Faraday effect interaction occurring between the light waves and symmetrically disposed dual segments of the compensating bias cell system, the segments being specially oriented so as substantially to compensate for or to eliminate undesired reactions in the cell to the presence of disturbing ambient magnetic fields. The compensating bias cell may be employed as a corner mirror in a ring laser device of the type used for measuring rotation rate.

United Sta Macek Mar. 14, 1972 [54] COMPENSATED FREQUENCY BIASING3,480,878 11/1969 Macek ..33 1/945 SYSTEM FOR RING LASER 3,486,13012/1969 Macek ..33l/94.5 3,508,831 4/1970 Macek ..33l/94.5 [72]Inventor: Warren M. Macek, Huntington Station,

N.Y. FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Sperry RandCorporation, Great Neck, 1,170,540 1 1/1969 Great Britain ..324/96 1Primary Examiner-Ronald L. Wibert [22] Flled' 1969 Assistant Examiner-R..l. Webster [21 Appl. No.: 873,257 y Yeawn [57] ABSTRACT [52] US. Cl...33l/94.5, 356/106 LR, 324/43,

224 9 3 0 47 3 151 3 0 57 3 0 A compensated Faraday bias cell system isadapted to sh1ft (bf- 51 1111. C1 ..G02r1 22,1-101s 3/10 feremially therespective Pbases 0f the contradirecfionauy [58 Field of Search..331/94.5; 356/106 RL, 117; W light Y Propagating in a ring laser G n"T 324/43 L 96; 350/147, 150, 151 157 160 deslred phase sh1ft 1s producedby Faraday efiect mteractlon occurring between the light waves andsymmetrically disposed [56] References Cited dual segments of thecompensating bias cell system, the seg- 'ments being specially orientedso as substantially to compen- UNITED STATES PATENTS sate for or toeliminate undesired reactions in the cell to the presence of disturbingambient magnetic fields. The compenl,96l,706 6/1934 Pajes ..350/151 micell may be employed as a comelmirror in a ring 3,360,323 12/1967we'sman laser device of the type used for measuring rotation rate.3,392,622 7/1968 Senf 3,462,708 8/1969 McClure ..331/94.5 10 Claims, 4Drawing Figures 16 52 Q A), a

PATENTEDMARM ma 3,649,931

' sum 1 [IF 2 00w cw 19 I l FREQUENCY 21 METER f FIG.4.

I/V VE/V rm? WA RRE/v M. MA GEK ATTORNEY COMPENSATED FREQUENCY BIASINGSYSTEM FOR RING LASER BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to ring lasers and moreparticularly to means for differential phase shifting of thecontradirectional light waves propagating in a ring laser, a phase shiftneeded for producing a discrete difference between the respectivefrequencies of the respective light waves and thereby precludingundesired mode locking effects.

2. Description of the Prior Art A ring laser comprises an active lasingmedium disposed relative to reflective or refractive optical-cavityforming components adapted to direct light waves emitted from the activemedium in opposite directions around a closed planar path. Oscillatorymodes occur at those particular frequencies for which the closed pathlength is an integral number of light wave lengths. Hence, thecontradirectionally propagating light waves oscillate at the samefrequency when their respective path lengths are equal, but at differentfrequencies when the path lengths are unequal. The latter event occurs,for example, when the cavity is rotated about an axis perpendicular tothe propagation plane of the light waves. The rotational rate may bemeasured by extracting from the cavity a small portion of the energy ineach light wave by partial transmission through or reflection from oneof the cavity-forming components. Combining means external of the cavitydirects the extracted light waves in collinear relation onto aphotodetector which provides an electrical beat-frequency signal whosefrequency corresponds to the difference between the light wavefrequencies. The difference frequency is linearly related to therotation rate of the ring for comparatively fast rotation but, as therate decreases toward zero, the relationship becomes nonlinear becauseof coupling between each light wave within the ring of backscatteredcomponents of the oppositely propagating waves. As the rotation ratedecreases even farther toward zero, but while still at some finitevalue, the coupling becomes sufficient to synchronize thecontradirectional waves, resulting in an abrupt cessation of thebeat-frequency signal. This frequency synchronizing phenomenon isreferred to as mode locking and the corresponding beat frequency orrotational rate at which it occurs is called the mode locking threshold.

To avoid mode locking and the consequent inability of the ring laser tosense rotational rates in its presence, a nonreciprocal phase shift mustbe imparted to the waves either by rotating the ring in the aforesaidmanner or by inserting in the propagation path some means such as anoptically birefringent medium that exhibits discrete propagationconstants to orthogonally polarized waves propagating through it. Thus,if the contradirectional waves are orthogonally polarized whiletraversing the birefringent material, their effective closed pathlengths will be unequal. As a result, the waves oscillate at differentfrequencies and, if the difference frequency is sufficiently large, modelocking will not occur. Under these conditions, rotation of the ringlaser will either increase or decrease the beat frequency, therebyproviding an indication of both rate and sense of rotation.

Although prior art frequency biasing cells comprising birefringentdevices used in conjunction with polarization converters or rotatorshave been successful in avoiding mode locking, they have also createdother problems which detract from their utility in some circumstances.More specifically, use of such frequency bias cells increases the costof the rotation sensor and makes it more difficult to align. Inaddition, transmission types of bias cells increase backscatter which,in turn, may increase the coupling between the contradirectionallyflowing waves, causing the mode locking threshold to increase. Thisundesirably reduces the dynamic rotation rate sensing range asdetermined by the difference between the nominal beat frequency and thelocking frequency.

Furthermore, prior art transmission types of Faraday bias cells areplaced within the ring itself and play only the role of bias control.Location of the bias cell within the ring limits flexibility of designand. injects troublesome and complex requirements as to temperaturestabilization and shielding against the effects of any ambient magneticfields that may be present. The conventional transmission type ofFaraday bias cell is inherently a magnetic field sensitive device, andthus must be adequately shielded against stray magnetic fields. Suchfields shift the bias point, thus undesirably disturbing the calibrationof the rate of turn device. If significant magnetic field time or spacegradientsare present in the volume occupied by the ring, the output ofthe device is correspondingly disturbed. To avoid such disturbances,prior art systems require the use of bulky, shielding, undesirablyincreasing the cost and weight of the instrument.

SUMMARY OF THE INVENTION The present invention provides means fordifferentially phase shifting the contradirectionally flowing waves in aring laser to produce a frequency bias without the necessity for eitherrotating the ring or inserting active components into the lightpropagation path itself. Operation of the inventive apparatus is basedon the classical Faraday effect. It is well known that a materialexhibiting the Faraday effect will in the presence of a magnetic fieldcause a nonreciprocal phase shift to occur between circularly polarizedlight waves where each is polarized in the opposite sense.

As in the conventional ring laser transmission Faraday cell geometry,the plane polarized light waves pass through a properly orientedquarter-wave plate and then into the Faraday The plane polarized lightpassing into the quarterwave plate is converted into circularlypolarized light. When it emerges from the Faraday material, it isconverted back into a plane polarized wave through the operation of asecond quarter-wave plate.

In the invention, two independent Faraday cell segments or units, eachsomewhat similar to the above-described unit, are employed. Each isexcited by a separate magnetic field generating solenoid. The cell unitsare oriented so that the light beam paths through them are substantiallyparallel. The light path from the output of the first cell segment orunit to the input of the second is completed by a mirror or otherreflective system. In the second cell unit, the light waves once againexperience the operation of the Faraday effect; thus, the total usefulphase shift is, for example, double that effected by one of theindividual cell units.

0n the other hand, at least the first order effect of any ambientmagnetic field present is cancelled by virtue of the specialside-by-side orientation of the two Faraday cell units. For example, anysuch disturbing field will add to the desired phaseshift produced by thefirst cell unit, but will in the second identical cell unit simplysubtract the same phase shift from the total desired phase shift. Thus,perturbing ambient magnetic fields have substantially no effect on thedesired total Faraday phase shift experienced by light waves propagatingin either sense through the bias device.

In addition to the important feature of the invention that the effectsof perturbing magnetic fields are substantially cancelled, the inventionis characterized by other important aspects. It is seen that theinvention has the properties of the simple corner mirror of whichseveral are commonly employed in ring laser devices. It may thus beemployed as a substitute for one of the conventionally employed simplemirrors. Thus, the Faraday magneto-optic effect may be used to frequencybias a ring laser while at the same time providing a magneticallycontrolled cavity-forming corner mirror. This is accomplished byconstructing a corner reflector or mirror including Faraday effect cellsin which opposed magnetic fields are established preferably in adirection lying in the plane of the ring laser cavity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a ring laseroptical cavity incorporating the invention;

FIG. 2 is a plan view in partial cross section of a preferred embodimentofthe invention;

FIG. 3 is a schematic diagram of a circuit for exciting magnetic fieldsused in the invention; and

FIG. 4 is a fragmentary schematic plan view of an altemative form of apart of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, an activelasing medium, such as the standard He-Ne gas mixture energized byconventional r.f. means (not shown), contained within glass tube emitslight waves in both directions along its longitudinal axis through optical flats l1 and 12 sealing the ends of the tube. Optical cavity-forming mirrors 13, 14, 15 and 16 successively reflect thecontradirectionally flowing light waves around a closed path 17. Theoptical flats are inclined at Brewsters angle with respect to thelongitudinal axis of tube 10 to provide light waves that are planepolarized parallel to the plane of the optical cavity, light sopolarized being referred to hereinafter as horizontally polarized.Although other polarization orientations may be used, the light wavesare preferably horizontally polarized in practicing the invention inorder to minimize distortion; that is, to prevent the light frombecoming elliptically polarized upon being reflected from thecavity-forming mirrors, as will be explained subsequently in greaterdetail.

A measure of the difference between the frequencies of thecontradirectionally flowing waves resulting from non-reciprocal effectspresent in the optical cavity is obtained by transmitting part of thelight energy in each beam through comer mirror 15 to a combinermechanism comprising mirrors l8 and 19, beam splitter 20, andphotodetector 21. The component of the clockwise light wave transmittedthrough corner mirror 15 is first reflected from combiner mirror 19 andthen partially reflected from beam splitter 20 into the photodetector.Likewise, the portion of the counterclockwise wave extracted from thecavity is reflected from combiner mirror 18 and partially transmittedthrough the beam splitter 20 in collinear relation with the clockwisewave into photodetector 21, wherein the light waves mix to produce abeat frequency signal whose frequency is equal to the difference betweenthe wave frequencies. The beat frequency signal may then be read on acommercially available frequency meter 22.

To overcome the mode locking effect which occurs at low rotationalrates, a non-reciprocal phase shifting device is incorporated in theoptical cavity by means of a magnetic field established, for example, incorner mirror system 16. The magnetization in the comer mirror systeminteracts with the incident contradirectionally flowing waves in such amanner that a differential phase shift is imparted to the waves in theprocess of being reflected from the mirror system. As a result, thecontradirectional waves oscillate at different frequencies so that modelocking does not occur even when the optical cavity is stationary. Themagnetization in the comer mirror system is preferably oriented parallelto the plane of the optical cavity. These conditions of polarization andmagnetization provide the desired differential phase shift whilesimultaneously preserving the polarization of the light waves.

It should be understood that the term ring as employed in the foregoingdiscussion and in what follows refers not to a circular ring, but to anN-sided figure, where the value of N is usually an integer such as threeor four. For example, FIG. 1 has been drawn to illustrate the inventionas operating in a four-sided or square ring. In FIG. 2, from which thestructure and operation of the inventive composite mirror-optical phaseshifter will be described, it will be explained that the compositemirror-phase shifter may be employed as previously suggested in theposition of corner mirror 16 of FIG. 1 or that it may be used in asimilar way in a laser ring having only three sides. The structure andoperation of the composite mirroroptical phase shifter apparatus shownin FIG. 2 is the same in both instances, with the mere exception thatthe angles of entry and exit of the light waves into the compositemirrorphase shifter structure 16 will differ, as will be described inwhat follows.

Referring now to FIG. 2, there is illustrated a composite mirror-phaseshifter device 16 for employment in place of the conventional type ofsimpler mirror that has been used in the past in ring lasers. The device16 includes a light beam direction means 1 illustrated in FIG. 2 asconsisting of symmetrically oriented prisms 20 and 120. The right angleprisms 20, 120 are placed so that they form a configuration with mirrorimage symmetry and, thus, so that they make contact along a single line21 perpendicular to the plane of the drawing, the base sides 22 and 122of the respective prisms being coplanar. Angles A and A are The valueschosen for the other angles of prisms 20 and depend upon the anglesbetween the respective exit and entry paths 23 and 123 of the lightbeams. In any event, the angle B between, for example, the ray path 23and the perpendicular 24 to the entrance sur face 25 of prism 20 ispreferably selected to be equal to Brewsters angle so as to permitefficient transfer of the light waves along path 23 into prism 20. Thesimilarly defined angle B associated with the ray path 123 and prism 120is similarly made equal to Brewsters angle.

Prisms 20 and 120 are cemented to an optical quarter-wave plate 30 whosefunction will hereinafter be described. Prisms 20 and 120 are cementedalong one surface of the quarter wave plate 30 by employing a layer ofconventional optical cement 31 of the type often used in the art forbonding optical components to each other. The cement layer 31 mayconstitute any of several lens bonding cements available on the market;for example, a cement of the epoxy type is useful. Generally, the cementemployed has an index of refraction intennediate that of the material ofprisms 20 and 120 and the material of quarter-wave plate 30.

Built upon the quarter-wave plate 30 and including it is the opticalphase shifting device 2 including elements 32 and 132. Phase shifterelement 32, for example, comprises a rod or cylinder 32 of a materialhaving a suitable Faraday coefficient and otherwise being mechanicallyand physically suitable for the application. The rod 32 is polished atits end 33 and is optically cemented at juncture 36 to the upper surfaceof quarterwave plate 30.

Any of a variety of known materials, including certain glasses, may beemployed in fabricating rod 32, such as certain glass materials known,for example, as extra light flint glass. Other glass materials, such asbarium light flint, may be employed. Rod 32 has an end 34 opposite end33 which is also ground to be optically flat and, furthermore, isparallel to end 33. Rod 32 is encompassed by a solenoid 35 wound with asufficient number of turns of conducting wire such that a magnetic fieldon the order of 100 Gauss may be generated within the Faraday phaseshifter rod 32.

Continuing a construction having mirror image symmetry, a second Faradayphase shift rod 132 with an optically flat end 133 is cemented atthejuncture 136 to quarterwave plate 30. Like phase shifter rod 32, rod132 has a second end 134 opposite its first end 133 which is groundoptically flat and parallel to end 133, and rod 132 is furthermoreprovided with an encompassing solenoid 135 having capabilities similarto solenoid 35.

The dual phase shifter section 2 of the composite mirrorphase shifterdevice 16 is completed by the use of a second quarter-wave plate 40.Quarter-wave plate 40 is placed on top of the respective ends 34 and 134of Faraday phase shifter rods 32 and 132. Quarter-wave plate 40 isfurthermore cemented to the respective ends 34 and 134 of phase shifterrods 32 and 132. Again, the cement employed between surfaces 34 and 134and the optically flat surface of the second quarterwave plate 40 isapplied in such a manner that thejunctures 41 and M1 have an index of arefraction intermediate those of the Faraday rods 32 and 132 and theindex of a refraction of quarter-wave plate 40.

Quarter-wave plates 30 and 40 are manufactured with optically ground andparallel upper and lower surfaces and may be made of any of severalsuitable materials in any of several suitable ways as characterized byany of a variety of conventional quarter wave devices available on themarket. Generally, the most useful material is crystalline quartz and itis preferable to use the conventional zero-order type of quarter-waveplate construction. Thus, quarter-wave plates 30 and 40 are ofthicknesses suitable for handling and for bonding to rods 32 and 132 insuch a manner that the composite structure 16 is reasonably easy tofabricate and has acceptable structural properties.

The composite mirror-phase shifter system 16 is completed by an element3 which may employ refractive and/or reflective elements. As notedpreviously, its function is to return light waves emanating from thedual phase shifting means 2 back into said means 3. The means 3comprises a simple equilateral prism 50 having reflective sides 51 and151 which may be coated, for example, with a thin layer of gold or otheroptically reflecting material, such as at 52 and 152. The angles C and Cof the prism may be arranged so that the light waves strike the surface51 in such a manner that total internal reflection is obtained. Prism 50may be constituted of a suitable flint glass or other material of whichsuch equilateral roof reflector prisms are conventionally made.Furthermore, it is bonded at juncture 53 by a suitable optical bondingcement to the upper surface of quarter-wave plate 40, that upper surfaceand the surface 53 of prism 50 to be bonded having been ground so as tobe optically flat.

In operation, it is observed that light waves enter and leave thecomposite mirror-phase shifter structure 16 both along light paths 23and 123. In the instance of a four-sided ring laser, the angle D betweenpaths 23 and 123 will generally be 90. Should a three-sided ring beemployed, as is suggested in FIG. 2, the angle E associated with mirror13' and the angle E associated with mirror 15 may both be made equal,for example, to 67. In this instance, the angle D will be 48. In anyevent, for any selected value of angle D, the angle B characterizingpath 23 relative to the surface 25 of prism 20 will preferably be theBrewsters angle.

Consider now an optical wave 23 travelling into the surface 25 asindicated by the arrow a. As aforementioned, wave 23 is horizontallypolarized, i.e., polarized in the plane of the drawing as indicated bythe double headed arrow 60. Wave a strikes surface 25 and is refractedinto prism 20, retaining its horizontal polarization characteristic. Intraversing quarterwave plate 30, its linearly polarized characteristicis converted into circular polarization, whereupon it flows into theFaraday rod 32.

Within rod 32, the waves direction of propagation is indicated by thearrow a, and its sense of polarization is indicated in the conventionalmanner for indicating a circularly polarized light wave by the arcuatearrow 61. Within rod 32, the light wave suffers a phase shift inproportion to the magnetic field H, generated by solenoid 35.

Upon leaving rod 32 at surface 34, the light wave passes throughquarter-wave plate 40 which, as is well known, has the property ofconverting the circular polarization 61 back into the horizontallypolarized state. Passing into reflector prism 50, the wave underdiscussion is propagated as illustrated by the arrow a and remainshorizontally polarized as indicated by the double headed arrow 62 afterreflection at point 64 from reflecting surface 51. The wave 0 travelshorizontally through prism 50, then again experiences total reflectionat point 164 on reflecting surface 151 of prism 50. The wave reflectedat point 164 passes perpendicularly through quarterwave plate 40 whereit again experiences conversion from a linearly polarized state to acircularly polarized state such as indicated by the arcuate arrow 65adjacent arrow (1 which latter defines the direction of flow of the waveunder discussion in the Faraday rod 132. Within rod 132, the wave aexpen'ences a phase shift in proportion to the magnitude of magneticfield H In one mode of operation of the apparatus, the magnetic fieldsH, and H are chosen to be equal, but are directed in opposite senses.However, these fields appear in this mode to be in the same sense as faras the light wave a is concerned as it traverses through them. The phaseshift undergone by wave a is therefore equal in both rods and in totalis equal to twice the phase shift in the rod 32, for example. Such aresult is further enhanced by making the rods 32 and 132 and theassociated optical parts of the invention have mirror image symmetry.

Wave a; now passes out of rod 132 and again flows through quarter-waveplate 30, wherein it experiences conversion into a plane polarized wave.That wave traverses prism 120, wherein it is refracted at surface asdescribed previously. Thus, the emergent wave a is shown as beinglinearly polarized in the plane of the drawing by the double headedarrow 66. Wave a is reflected by corner mirror 15 into laser 10' andflows out of laser 10 in an amplified state to be reflected by comermirror 13' from which it originated.

The operation of the system is similar for the counterclockwise wave breflected by comer mirror 15 to be refracted into prism 120 at surface125. Note that wave b traverses the same path 23, 123 as was traversedby wave a andis horizontally polarized as is indicated by the doubleheaded arrow 66. Upon entry into quarter-wave plate 30, the wave becomescircularly polarized. Its state is indicated by the arcuate arrow 67adjacent the arrow b,, the latter indicating wave b s direction of flowin rod 132. Observe that it is now flowing in the direction of field HUpon emergence from rod 132, wave b, undergoes conversion to linearlypolarized energy. After reflection at point 164, its direction of flowis indicated by the arrow b and its sense of polarization again by thedouble headed arrow 62. Now, the wave h is reflected at point 64; againfollowing the same path as was followed by the previously described ray,it reenters quarter-wave plate 40. Here, it is converted again intocircularly polarized energy as indicated by the arcuate arrow 68, itssense of flow being shown by the arrow b Note again that the directionof flow is the same as the direction of magnetic field H in rod 32.

Wave b experiences a phase shift in rod 32 in proportion to themagnitude of field H whereupon it emerges into quarterwave plate 30,again being converted into horizontally polarized energy. Afterrefraction at surface 25 of prism 20, the wave is represented as to thedirection of its propagation by the arrow 11., and as to character oflinear polarization again by the double headed arrow 60. The wave bcontinues to travel along path 23 and is reflected by comer mirror 13'through laser 10 which projects it after amplification onto mirror 15from which it originally started.

It is a feature of the invention that the respective solenoids 35 andassociated with rods 32 and 132 are wound in such amanner that themagnetic fields H H cause the wave a to be shifted in phase by two equalincrements in one sense. On the other hand, the fields H and H are seento cause the wave 11, to be shifted by the same two increments of phaseshift, but in the opposite sense. Thus, the difference between thephases of waves a and b is equal to four times the phase shift produced,for instance, by rod 32 when wave a flows through it alone. 7

On the other hand, a feature of the invention is that any ambientmagnetic field, fixed or variable, which has a component in thedirection of the arrow labeled H produces equal phases shifts in rods 32and 132. However, the light waves a and b see these phase shifts asbeing in opposite senses, so that after the complete excursion throughthe composite mirror phase device 16, neither of the waves a, b arephase shifted because of the presence of said ambient field H Thus, thecomposite comer mirror phase shifter device is compensated for theundesired effects that are caused in a conventional single celltransmission type of Faraday phase shifter device by the presence ofambient magnetic fields.

Furthermore, this novel result is achieved in a structure which hasmirror image optical symmetry. It has long been an established principleof good optical design that refractive, reflective, and other opticalelements which have such symmetry are superior in that the presence ofsuch a characteristic makes such elements easier to use in an opticalsystem, problems with optical alignment being diminished. Furthermore,the magnetic field path tends to be a closed one and also has symmetryand is therefore more efficient. The arrangement forms a compactreflector-phase shifter device which may be placed at a corner of a ringlaser. it is therefore not placed directly within the ring and it may beplaced in relation to the ring mounting structure in such a way that itis relatively easily replaced and in such a manner that thermal problemsare reduced. Placement of the device in a corner of the ring permitsimproved thermal stability of the ring.

A further feature of the inventive concept may be made ap parent fromFIG. 3. Here, solenoid 35 is shown as supplied by a unidirectionalcurrent via leads 84 from battery 80 regulated by rheostat 82 whenswitch 81 is closed. Similarly, an equal current flows via leads 184 tosolenoid 135 when switch 81 is closed. With reversing switch 83 closedin one sense, the respective fields caused by solenoids 35 and 135 arein the respective senses of arrows H and H i.e., the fields H l and H,cause differential phase shift of the counterrotating light beams andproduce a desired frequency bias effect.

Should it be desired to operate the ring laser in a second mode withoutfrequency bias, but to be able to return instantaneously to the first orfrequency biased mode of operation, switch 83 is simply reversed, sothat solenoid 35 generates the field H Now, no differential phase shiftis produced, since there is no net effect of fields H, and H However,any ambient field H also cancels out, as before. In either mode, heatlosses are the same in solenoids 35 and 135 and there is no net adverseeffect caused by thermal drift and no consequent undesired transient orother effect on the calibration of the instrument.

FIG. 4 illustrates another versatile feature of the invention. Here aprism having modified light wave entrance and exit faces and 125 isillustrated. The light wave paths 23, 123, respectively similar to paths23, 123 of FIG. 2, cross at point 90 at an angle D, as before. The wavefollowing path 123', for example, is refracted at face 25 into prism 20and then passes through quarter-wave plate and along the electricallyactive direction of Faraday rod 32. Similarly, a wave following path 23'is refracted at face 125 into prism 20' and then passes throughquarter-wave plate 30 and along the electrically active direction ofFaraday rod 132. The reverse flowing waves follow the same paths,producing counterrotating circulating waves when applied in a laserring.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

What is claimed is:

1. Phase shifter apparatus for providing non-reciprocal differentialphase shift in counter-flowing light beams passing therethroughcomprising:

first and second magnetic-field-controllable optical phase shiftermeans,

said optical phase shifter means including non-reciprocal first andsecond means for shifting the phase offirst and second circularlypolarized light beams passing therethrough along respective first andsecond light beam transmission paths, each said means for shifting thephase of circularly polarized light beams having substantially parallelfirst and second ends,

said first ends being bonded to a first face of first quarterwave platemeans for reciprocal conversion of light between linearly and circularlypolarized states,

said second ends being bonded to a first face of second quarter-waveplate means for reciprocal conversion of light between linearly andcircularly polarized states, means for orienting said first and secondlight beam transmission paths substantially perpendicular to said respective first ends, light beam directing means for providing a third lightpath joining said first and second light paths for the purpose ofproviding a continuous light path, AND

first and second magnetic field generator means respectively coupled tosaid first and second magnetic-field-controL lable optical phaseshifting means for providing equal oppositely directed magnetic fieldsand said respective first and second light beam transmission paths andfor the purpose of producing differential phase shift in said lightbeams traveling along said light paths in opposite directions whilecancelling any phase shift effect thereon caused by the presence of anambient magnetic field.

2. Apparatus as described in claim 1 wherein said means for orientingsaid first and second light beam transmission paths substantiallyperpendicular to said respective first ends comprises a refractive prismbonded to a second face of said first means for reciprocal conversion oflight between linearly and circularly polarized states.

3. Apparatus as described in claim 1 wherein said light beam directingmeans for providing a third light path joining said first and saidsecond light paths for the purpose of providing a continuous light pathcomprises a reflecting prism bonded to a second face of said secondmeans for reciprocal conversion of light between linearly and circularlypolarized states.

4. Phase shifter apparatus for providing non-reciprocal differentialphase shift in counter-flowing light beams passing therethroughcomprising:

first and second magnetic-field-controllable non-reciprocal opticalphase shifting means aligned in substantially parallel relation forproviding substantially geometrically parallel first and second lightpaths,

light beam directing means for providing a third light path joining saidfirst and second light paths for the purpose of forming a continuouslight path, and

first and second magnetic field generator means respectively coupled tosaid first and second magnetic-field-controllable optical phase shiftingmeans for providing substantially equal oppositely directed magneticfields along said respective first and second light paths for thepurpose of producing differential phase shift in said light beamstraveling along said light paths in opposite directions while cancellingany phase shift effect thereon caused by the presence of an ambientmagnetic field.

5. Apparatus as in claim 4, wherein said first and second fieldcontrollable optical phase shifting means each include:

first and second means for reciprocal conversion of light betweenlinearly and circularly polarized states,

at least one means for shifting the phase of circularly polarized light,

said first means for reciprocal polarization conversion being bonded ata portion of one of its faces to one end of said means for shifting thephase of circularly polarized light, and

said second means for reciprocal polarization conversion being bonded ata portion of one of its faces to a second end of said means for shiftingthe phase of circularly polarized light.

6. The apparatus of claim 4 in combination with:

additional means external of said differential optical phase shifterapparatus for amplifying light waves propagating in opposite directionsthrough said optical phase shifter apparatus.

7. The apparatus of claim 6, wherein:

said additional means comprises:

at least a pair of light redirecting components adapted to formcomponents of a laser optical cavity, an active lasing medium positionedin said optical cavity.

8. Apparatus as in claim 7, wherein:

said light waves propagating in opposite directions through said opticalphase shifter apparatus propagates therein along paths lyingsubstantially in a plane, and

said light waves when propagating elsewhere in said optical cavity islinearly polarized in said plane.

9. Apparatus as described in claim 4 and further including:

electrical current source means,

first and second solenoid means respectively encompassing at least partsof said respective magnetic-field-controllable optical phase shiftermeans, and

1. Phase shifter apparatus for providing non-reciprocal differentialphase shift in counter-flowing light beams passing therethroughcomprising: first and second magnetic-field-controllable optical phaseshifter means, said optical phase shifter means including non-reciprocalfirst and second means for shifting the phase of first and secondcircularly polarized light beams passing therethrough along respectivefirst and second light beam transmission paths, each said means forshifting the phase of circularly polarized light beams havingsubstantially parallel first and second ends, said first ends beingbonded to a first face of first quarterwave plate means for reciprocalconversion of light between linearly and circularly polarized states,said second ends being bonded to a first face of second quarter-waveplate means for reciprocal conversion of light between linearly andcircularly polarized states, means for orienting said first and secondlight beam transmission paths substantially perpendicular to saidrespective first ends, light beam directing means for providing a thirdlight path joining said first and second light paths for the purpose ofproviding a continuous light path, and first and second magnetic fieldgenerator means respectively coupled to said first and secondmagnetic-field-controllable optical phase shifting means for providingequal oppositely directed magnetic fields and said respective first andsecond light bEam transmission paths and for the purpose of producingdifferential phase shift in said light beams traveling along said lightpaths in opposite directions while cancelling any phase shift effectthereon caused by the presence of an ambient magnetic field. 2.Apparatus as described in claim 1 wherein said means for orienting saidfirst and second light beam transmission paths substantiallyperpendicular to said respective first ends comprises a refractive prismbonded to a second face of said first means for reciprocal conversion oflight between linearly and circularly polarized states.
 3. Apparatus asdescribed in claim 1 wherein said light beam directing means forproviding a third light path joining said first and said second lightpaths for the purpose of providing a continuous light path comprises areflecting prism bonded to a second face of said second means forreciprocal conversion of light between linearly and circularly polarizedstates.
 4. Phase shifter apparatus for providing non-reciprocaldifferential phase shift in counter-flowing light beams passingtherethrough comprising: first and second magnetic-field-controllablenon-reciprocal optical phase shifting means aligned in substantiallyparallel relation for providing substantially geometrically parallelfirst and second light paths, light beam directing means for providing athird light path joining said first and second light paths for thepurpose of forming a continuous light path, and first and secondmagnetic field generator means respectively coupled to said first andsecond magnetic-field-controllable optical phase shifting means forproviding substantially equal oppositely directed magnetic fields alongsaid respective first and second light paths for the purpose ofproducing differential phase shift in said light beams traveling alongsaid light paths in opposite directions while cancelling any phase shifteffect thereon caused by the presence of an ambient magnetic field. 5.Apparatus as in claim 4, wherein said first and second fieldcontrollable optical phase shifting means each include: first and secondmeans for reciprocal conversion of light between linearly and circularlypolarized states, at least one means for shifting the phase ofcircularly polarized light, said first means for reciprocal polarizationconversion being bonded at a portion of one of its faces to one end ofsaid means for shifting the phase of circularly polarized light, andsaid second means for reciprocal polarization conversion being bonded ata portion of one of its faces to a second end of said means for shiftingthe phase of circularly polarized light.
 6. The apparatus of claim 4 incombination with: additional means external of said differential opticalphase shifter apparatus for amplifying light waves propagating inopposite directions through said optical phase shifter apparatus.
 7. Theapparatus of claim 6, wherein: said additional means comprises: at leasta pair of light redirecting components adapted to form components of alaser optical cavity, an active lasing medium positioned in said opticalcavity.
 8. Apparatus as in claim 7, wherein: said light wavespropagating in opposite directions through said optical phase shifterapparatus propagates therein along paths lying substantially in a plane,and said light waves when propagating elsewhere in said optical cavityis linearly polarized in said plane.
 9. Apparatus as described in claim4 and further including: electrical current source means, first andsecond solenoid means respectively encompassing at least parts of saidrespective magnetic-field-controllable optical phase shifter means, andreversible switch means having a normal and a second state, said switchmeans being coupled to said solenoid means and to said current sourcemeans for selectively reversing current flow in at least one of saidsolenoid means when in said second state.
 10. Apparatus as described inclaim 4 and further including Brewster angle prism means bonded to asecond face of said first means for reciprocal conversion of lightbetween linearly and circularly polarized states.